Generally, in a boiling water nuclear power plant, power control is achieved by control rod operation and recirculation flow control. Thus, a thermal state value is monitored by a core monitoring system. For example, such a core monitoring system performs the monitoring by combining a core performance calculating device that performs time consuming power distribution calculation and a core thermal limit value monitoring device that performs the power distribution calculation in a relatively short time.
The core performance calculating device takes long time to complete the power distribution calculation. Therefore, the core monitoring system that uses the combination of the core performance calculating device and core thermal limit value monitoring device performs monitoring by using, in a complementary manner, the core thermal limit value monitoring device that can obtain a calculation result in a relatively short time.
The calculation time of the core thermal limit value monitoring device is desirably short. However, requirement of calculation processing covering the entire monitoring target region of the core limits a calculation cycle to about 200 ms. The core is divided into a plurality of regions so as to be monitored by a computer having a plurality of CPUs (see, for example, Japanese Patent Application Laid-Open Publication No. 2007-278991, the entire content of which is incorporated herein by reference).
In the case where the core is divided into a plurality of regions so as to be monitored by the computer having a plurality of CPUs, a control unit determines output states of signals from all the CPUs. In this case, however, the control unit needs to determine the signal output state even for a CPU that need not output the signal, so that the control unit cannot perform processing according to the output signals from the core thermal limit value monitoring device, preventing a reduction in the entire calculation time.